Rockbox is an open source firmware project designed to work with several popular - though generally older - MP3 players, from iPods to irivers.

If you're looking for a good-but-cheap gift for the music-lovin' geek in your life, consider buying them an older iPod or other MP3 player and tweaking it out with Rockbox, or perhaps iPodLinux. If that special someone likes tinkering, the cheapie MP3 player with Rockbox or iPodLinux would make for a helluva gift. I've used both, and though I've found Rockbox to be a touch more difficult to install, the PDF manuals really hold your hand through the process.